Circuit-arrangement for protecting amplifiers from oyerload



March 16, 1954 J. ENSINK CIRC -ARRANGEMENT FOR PROTECTING PLIFIERS FROM OVERLOAD Filed Oct. 9

INVENTOR Johan es E Patented Mar. 16, 1954 GmQUIT-ABRAN EM T ,FQR RQ ECTIN PLIFIERfi F M QV LQAP Johannes .Ensink, Hilversum, Netherlan Qomrsna si ner t .ll rt erd nalBank and This? tford, Conn., as trustee Application pctober' 9, I951 Serial No. 250,468

pt erity, Qc

2 Claims. 1

.'.he invention relate to s lent-a ran ements or. rotectin amplifi r in r ticul l'y ga e -w e amplifiers. f eer-be e .qve tleeded.

An amplifier for the amplification of a carrierwave oscillation is frequently required to have a constant output uolta ge. For this purpose a negative feedback voltage which is proportional to the output voltage is supplied through a rectifierzhaving a threshold voltage to the input circuit. of the amplifier so that, if this negative v feedback voltage exceeds the threshold. voltage,

a substantial negative feedback preventing a further increase in output voltage becomes operative across the input circuit.

Under certain conditions it may occur that the output impedance of the amplifier is short-circuited or is at least considerably lower than that permissible for the amplifier. The normally short-circuited output circuit of a spare amplifier is, for example, connected in series with that of a main amplifier; this short-circuit being removed only when the main amplifier breaks down. Consequently, the shortcircuited amplifier will be overloaded, which might materially shorten its lifetime.

In order to avoid this overload, in accordance with the invention, provision is made of two negative feedback circuits, each comprising a rectifier having a threshold voltage, of which the first, when the threshold voltage is exceeded, feeds back to the input circuit of the amplifier a negative feedback voltage which is proportional to the output voltage, the second, on the contrary, a negative feedback voltage which is proportional to the output current, the ratio between the negative feedback voltage and the threshold voltage of the second negative feedback circuit then exceeding that of the first negative feedback circuit, when the permissible load of the amplifier is exceeded.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing.

Referring to the figure of the drawing, reference numeral l designates an amplifying tube for the amplification of carrier-wave oscillations which are supplied through an input circuit 2 to the grid of the tube 1. The output circuit of the tube I comprises an output transformer 3, which supplies a voltage to a load impedance 4. In order to stabilize the output voltage across the transformer 3, the voltage produced across a secondary winding 5 is supplied through a rectifier 6 having a threshold voltage source 1 to the input circuit 2, so that, if the voltage across the app at on Nethe ands winding 5 exceeds the threshold voltage of the source T1, the input. circuit 2 has supplied to it a negative feedback voltage which counteracts a further increase in output voltage. In this case thelower'endTof the circuit 2 is connected by way ,of the normally conductive rectifier 8 to a point of constant potential.

If .theimpedance 4 should assume an excessivelylow value, for example, if it were shortcircuited, the amplifying tube 1 would be in dan; gel of being overloaded, since then the negative feedback voltage, ".whicli had to be supplied through the rectifier and ,the threshold voltage source l to'the input circuit 2 is not developed. In order to avoid this overload, in accordance with the invention, a transformer 53 is connected in series with the transformer 3, this transformer 9 being traversed by the anode current of the amplifying tube i, the voltage across the secondary winding It! being supplied also through a rectifier II and the threshold voltage source 7 to the input circuit 2 of the amplifying tube l. The transformer ratios of the transformers 3 and 9 are chosen to be such that, if the permissible load of the tube l is exceeded owing to an excessively low value of the impedance 4, the voltage across the winding it exceeds that across the winding 5. Consequently, even before the negative voltage feedback across the rectifier 5 becomes operative, if the permissible load is exceeded, the current feedback across the rectifier II will become operative, so that a further increase in current passing through the amplifying tube is counteracted, irrespective of the voltage produced across the load impedance 4.

If desired, two separate threshold voltage sources may be provided, of which one is only operative across the circuit of the rectifier 6, the other across that of the rectifier H. In this case, if the permissible load is exceeded, the ratio between the voltage across the winding iii and the threshold voltage across the circuit of the rectifier II is, consequently, required to exceed that between the voltage across the winding 5 and the threshold voltage across the circuit of the rectifier 6.

The amplifier hitherto described is, for example, used as a spare amplifier in a carrier-wave telephone system, in which the normally shortcircuited output circuit is connected in series with that of a main amplifier. In order to air-- ford an indication of whether this spare amplifier itself is in good order, provision is made of the circuit elements 8, [4, I5, it and ii. If the tube l operates correctly, a positive voltage will be produced across a filter [4 included in the grid circuit of this tube by means of a rectifier 15, this voltage neutralizing the voltage of a negative bias voltage source is, the grid bias voltage, however, not being adapted to exceed the potential of the right-hand electrode of the rectifier 8. However, if across the anode circuit of the tube l neither a sufiicient alternating voltage nor a sufiicient alternating current is produced, the voltage across the filter l4 drops and hence the grid voltage of the tube 1, so

that an alarm relay I! included in the anode developed in said output circuit, the second network being coupled to the input circuit to apply thereto when its threshold is exceeded a negative feedback voltage proportional to the current produced in said output circuit, the ratio of the negative feedback voltage to the threshold voltage of the second network being greater than that of said first network when the permissible load on the amplifier is exceeded.

2. Protective apparatus for preventing overloading of a spare carrier wave amplifier having a signal input circuit and an output circuit connected in series with the output terminals of a main amplifier, the output circuit being normally short-circuited when the main amplifier is operative, said protective apparatus comprising first and second negative feedback networks coupled to said output circuit, each network including a rectifier and a threshold voltage source therefor, said first feedback network being coupled to the input circuit to apply thereto when the threshold is exceeded a negative feedback voltage proportional to the voltage developed in said output circuit, the second network being coupled to the input circuit to apply thereto when its threshold is exceeded a negative feedback voltage proportional to the current produced in said output circuit, the ratio of the negative feedback voltage to the threshold voltage of the second network being greater than that of said first network when the permissible load on the amplifier is exceeded.

JOHANNES ENSINK.

References Cited in the file of this patent. UNITED STATES PATENTS Number Name Date 2,318,061 Dailey May 4, 1943 2,561,049 Buys et al. July 17, 1951 

